From Wikipedia: Nimbus Mono is a monospaced typeface created by URW Studio in 1984, and eventually released under the GPL and AFPL (as Type 1 font for Ghostscript) in 1996 and LPPL in 2009. It features Normal, Bold, Italic, and Bold Italic weights, and is one of several freely licensed fonts offered by URW++. Although the characters are not exactly the same, Nimbus Mono has metrics that are very similar to Courier and Courier New. It is one of the Ghostscript fonts, free alternatives to 35 basic PostScript fonts (which include Courier). It is a standard typeface in many Linux distributions.
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